Problem #3 A rock is thrown vertically upward with a speed of 12.0 m/s from the

Question

Problem #3
A rock is thrown vertically upward with a speed of 12.0 m/s from the roof of a building that is 60.0 m above the ground.
a. Sketch a position vs. time graph.
b. Sketch a velocity vs. time graph.
c. In how many seconds after being thrown does the rock strike the ground?
d. What is the velocity of the rock just before it strikes the ground?

Explanation / Answer

(i) By s = ut + 1/2gt^2 =>60 = - 12t + 1/2 x 9.8 x t^2 =>4.9t^2 -12t -60 = 0 =>t = [-(-12) +/-v{(-12)^2 - 4 x 4.9 x (-60)}]/[2 x 4.9] =>t = [12+/- 36.33]/9.8 =>t = 4.93 sec (ii) By v = u + gt =>v = -12 + 9.8 x 4.93 =>v = 36.33 m/s [downward]

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